Horst-Herzhorn is an Amt (collective municipality) in the district of Steinburg, in Schleswig-Holstein, Germany. The seat of the Amt is in Horst. Before 1 January 2008, when it was merged with the Amt Herzhorn, it was named Amt Horst.

The Amt Horst-Herzhorn consists of the following municipalities:

1. Altenmoor
2. Blomesche Wildnis
3. Borsfleth
4. Engelbrechtsche Wildnis
5. Herzhorn
6. Hohenfelde
7. Horst
8. Kiebitzreihe
9. Kollmar
10. Krempdorf
11. Neuendorf bei Elmshorn
12. Sommerland

53°49′01″N 9°37′01″E / 53.817°N 9.617°E / 53.817; 9.617